Photoelectric or photoemissive detectors operate on the photoelectric effect, which is a process in which electrons are liberated from a material surface after absorbing energy from a photon. All the energy from a photon is transferred to a single electron in a metal, and that electron, having acquired the energy in excess of Ew, will emerge from the metal surface with a kinetic energy given by
Einstein’s photoelectric equation
where v is the velocity of the emerging electron, m is the mass of an electron, and Ew is the work function. Ew is the energy required by an electron at the Fermi level to leave the metal surface with the highest kinetic energy.
A photoemissive detector is the basic component of vacuum or gas phototubes and photomultiplier tubes. A phototube consists of a semicircular photoemissive cathode concentric with a central rodlike anode in a special glass enclosure. The incoming photons impinge on the photocathode and generate photoelectrons which leave the cathode surface, and because of the electric field between the anode and cathode, these electrons are collected by the anode. This results in an anode current that is proportional to the number of photons impinging on the cathode.
The photomultiplier tube (PMT) operates on the same basic transduction mechanism as the phototube, but the PMT has provisions for realizing high current gain by current multiplication. The components of the PMT are shown schematically in Fig. 5.37.
|Figure 5.37 Photomultiplier tube.|
Photoelectrons are generated as in a phototube. These photoelectrons are focused onto the first element of the PMT chain, known as a dynode.
The photoelectrons from the first dynode are directed in succession to other dynodes, which are at progressively higher potential. The electrons accelerate toward these dynodes, and on striking them, they generate many more electrons by a process known as secondary emission. As an example, if there are 10 stages and if each primary electron produced 4 secondary electrons, then the total number of electrons at the end of the dynode chain would be 410 or approximately 106 electrons.
The electrons from the last dynode stage are collected by the anode.The resulting amplified current is proportional to the number of input photons.